Fiber-filled plastic compounds suitable for processing, such as by injection molding, have become widely used. The fibers impart many valuable characteristics to the articles so produced, foremost of which are high dimensional stability, high modulus of elasticity, high resistance to distortion by heat, high tensile strength, unusually high flexural modulus and low shrinkage during curing.
For example, thermoplastic injection molding compounds comprising bundles of glass fibers surrounded by thermoplastics and injection molding processes employing them are described in Bradt, U.S. Pat. No. 2,877,501. The Bradt patent discloses pellets comprising 15-60 wt. % glass in thermoplastic resin, e.g., polystyrene. This corresponds to 8.1%-42.9% of filaments by volume and correspondingly 91.9%-57.1% by volume of resin. Das and Moore, U.S. Pat. No. 4,477,496 disclose sized glass fiber bundles for reinforcing thermoplastics, but they do not lend themselves to injection molding. The Das et al patent disclosed using a plurality of sized glass fiber strands gathered, dried and collected for use as chopped strands for reinforcing polymeric materials. The sizing comprises a crosslinkable epoxidized thermoplastic film forming copolymer.
The technology of the Bradt patent has subsequently been extended. In addition to the styrene resins, styrene-acrylonitrile copolymer resins and styrene-butadiene copolymer resins described therein, numerous other injection-moldable thermoplastic resins, such as polycarbonate resins, acrylonitrile-butadiene-styrene terpolymer resins, poly(ethylene terephthalate) resins, polysulfone resins, polyphenylene ether resins, polyetherimide resins, nylon resins, and the like, have been effectively reinforced by glass fibers.
Moreover, instead of glass fibers, subsequently developed commercial products are reinforced with filaments of carbon fibers, graphite fibers, aramid fibers, stainless steel filaments and other, as well as mixtures of any of the foregoing, many such products stemming directly from the technology disclosed in the above-mentioned U.S. Pat. No. 2,877,501. Such technology involves providing elongated granules, each of the granules containing a bundle of elongated reinforcing filaments and a thermoplastic molding composition binding the bundle. In the process of injection molding, such granules are forced into a mold, wherein the filaments are dispersed and produce molded articles with improved properties in comparison with the molded thermoplastic alone.
Prior art processes for making filament-filled granules generally require a compounding/pelleting step, in which the thermoplastic material is mixed with filaments, usually in the form of chopped bundles of filaments, and usually in an extruder. The extrudate is then chopped and formed into molding granules. Such equipment is not readily available to the molder, and a number of specialty compounders have established businesses in which fibers from one source, and thermoplastics from another source are formulated into such granules in drums or truckloads for sale to molders. It is desirable for such molders to by-pass the need for such compounders and permit molders to feed mixtures of thermoplastics and fibers directly into the molding press hopper achieving fiber dispersion by shear forces at the screw, nozzle, check valve, runners, gates, etc., in the injection molding machine. It would also be desirable to use very little resin in the pellets and high filament loadings while maintaining fiber bundle integrity and ready dispersibility. The fibers of the filament bundles should not separate during chopping and tumbling with the reduced volume fractions or resin. Moreover, there should be little tendency to degrade if the temperature is raised to lower viscosity and thereby enhance fiber dispersion. In addition, individual fibers should not become airborne and cause problems in handling due to premature breakage of the granule.
In copending commonly assigned U.S. patent application Ser. No. 233,582, filed Aug. 18, 1988, now U.S. Pat. No. 4,944,965 are disclosed improved elongated granules which solve such problems by substituting for the thermoplastic matrix which separates, coats and binds the fiber bundles as in the Bradt patent a much thinner layer of an efficient thermoplastic adhesive. In contrast to the crosslinkable film-forming adhesive to the Das et al patent, which is intended to be incompatible with the thermoplastic to be reinforced, a compatible, wholly uncrosslinkable thermoplastic adhesive is to be used in the above-identified application as the binder resin. A binder resin composition particularly exemplified in the copending application is a poly(ethyl oxazoline). While this is generally satisfactory to hold the bundles together during chopping and transporting to the blender and injection molding machine, it has been observed that the granules could, under certain handling conditions, break apart during vacuum transport, which is a method very commonly used to move the elongated granules from storage to the injection molding machine feed hopper. The breaking of the granules leads to non-uniform distribution of the fibers in the ultimate molded article, and this in turn leads to low reinforcement of said molded article. Further, when metal fibers or metal-coated fibers are used, reduced attenuation of electromagnetic interference results, an undesirable result if the molded article is to be used in an application requiring its electrical shielding ability.
In commonly assigned co-pending U.S. Ser. No. 07/344,797, filed Apr. 26, 1989, now abandoned, are also disclosed improved elongated granules containing a film-forming thermoplastic polycarbonate binder. While these granules constituted an improvement over those of the prior art, their utility is limited to incorporation into thermoplastic resins which are compatible with the polycarbonate binder. Further, application of the polycarbonate resin to the fibers during granule production requires additional processing steps which should be avoided, if possible, such as the use and recovery of chlorinated organic solvents.
In accordance with the present invention, superior latex binder compositions have now been found. These will bind the fiber bundle together sufficiently to prevent said bundles breaking during chopping into pellets and tumbling with granules of the resin to be reinforced.
As will be seen, the binder resin compositions of this invention permit the molding process itself to be used to disperse the resin uniformly throughout the molded article thus avoiding the compounding/pelletizing step.
Moreover, through of the present invention greater and more uniform dispersions of the fibers are achieved, thereby permitting superior electromagnetic shielding to be obtained at equal load levels when using electrically conductive fibers, such as nickel coated graphite fibers. Further, the use of a latex binder simplifies the processing steps of application of the binder composition to the fibers since it involves the use of an aqueous dispersion.